1) Field of the Invention
The present invention relates to an optical device suitable for use with a case wherein monitoring light for monitoring optical modulation operation is fetched.
2) Description of the Related Art
In recent years, an optical modulator configuring an optical device has been applied to and used in a high-speed long-haul optical communication transmission apparatus. The optical modulator performs optical modulation by applying a modulation signal voltage to an optical waveguide formed on a substrate. As one of optical waveguides for optical modulation, a Mach-Zehnder (hereinafter referred to simply as MZ) type optical waveguide is known.
In the optical modulator as an optical device to which the MZ type optical waveguide is applied, a traveling wave electrode (electric waveguide) for controlling the relative phase of lights propagated along each of arm waveguides which form the MZ type optical waveguide is formed. In particular, a modulation signal voltage is applied to each traveling wave electrode to control the refraction index of the arm thereby to vary the optical path length difference between the two arms to achieve optical modulation.
It is to be noted that, in order to use a modulator having such a configuration as described above to obtain a suitable optical modulation signal, application of an RF modulation signal having a suitable voltage to the arm waveguides and application (operation point control) of a suitable DC bias voltage for controlling the relative phase shift amount between the two arm waveguides are demanded. Particularly, in order to suitably perform the latter operation point control, it is necessary to accurately monitor the optical output signal.
To this end, generally a function for monitoring the optical output signal is integrated in the optical modulator. However, two kinds of methods are generally available as a method for monitoring such suitable output signal light as described above. One of the methods is a technique (a) wherein modulated output signal light (main signal) itself is monitored, and the other one of the methods is a technique (b) wherein output signal light is monitored indirectly, not from the main signal itself but from light which has some correlation to the main signal.
As the former technique, three methods are available; a method (a-i) wherein a tap is provided for the optical waveguide to branch and monitor the main signal, another method (a-ii) wherein a half mirror or the like is arranged for the main signal after outputted from an optical substrate to branch and monitor the main signal, and a further method (a-iii) wherein leakage light from the main signal waveguide is picked up.
As the latter technique two methods are available; a method (b-i) wherein light which leaks into the substrate when phase-modulated light of the arms of the MZ optical waveguide are coupled is monitored, and another method (b-ii) wherein a portion for coupling phase-modulated light of the arms of the MZ optical waveguide is formed from an MMI (Multi-Mode Interferometer) or the like and performs switching operation such that an output on one side is used as monitor light. In short, according to the technique, two light fluxes having substantially reverse phases to each other are switched by the MMI or the like so as to be outputted alternately through the two waveguides, and one of the light fluxes is fetched as output signal light while the other one of the light fluxes is fetched as monitor light.
In the former technique, it is a prerequisite that the phase difference θ=0+α between the main signal and the monitor light is substantially 0 (that is, α→0) in order that the operation for applying a suitable DC bias voltage functions. Meanwhile, in the latter technique, it is a prerequisite that the main signal light (output signal light) and the monitor light have reverse phases to each other and the phase difference θ=n+α between the main signal and the monitor light is substantially n (that is, α→0), for example, as seen in FIG. 4 in order that the operation for applying a suitable DC bias voltage functions. In other words, a modulation state of the main signal light can be monitored accurately from the monitor light under such a phase relationship as described above.
As an example, FIG. 5 is a view showing an example of a configuration of an optical modulator 100 as an optical device to which the method (b-ii) described hereinabove is applied. The optical modulator 100 shown in FIG. 5 includes a substrate 191 on which an MZ type optical waveguide 110 and an electrode 111 are formed, a light reception section 121 for receiving monitor light, a modulation electric signal generation section 123 for generating an electric signal in accordance with modulation data to be supplied to the electrode 111, a bias voltage generation section 124 for generating an operation point voltage regarding a modulation electric signal to be supplied to the electrode 111, and a control section 125 for controlling the operation point voltage to be generated by the bias voltage generation section 124 in response to the monitor light received by the light reception section 121.
The MZ type optical waveguide 110 includes an input waveguide 101 for receiving input light, an MMI 102 connected to the input waveguide 101, two arm waveguides 103 branched at the MMI 102, another MMI 104 connected to the two arm waveguides 103, and an output optical waveguide 105 and a monitoring optical waveguide 106 further branched at and connected to the MMI 104 after the two arm waveguides 103 are coupled at the MMI 104.
Output signal light propagated along the output optical waveguide 105 is outputted from a face opposite to that of the substrate 191 to the input light is inputted. Further, a reflection groove 113 is formed at the downstream side end of the monitoring optical waveguide 106, and light reflected on the reflection groove 113 is outputted from a side face of the substrate 191 different from the end face from which the output signal light is outputted. In particular, the light reception section 121 is formed on the side face side of the substrate 191 and receives the light propagated through the monitoring optical waveguide 106 and reflected by the reflection groove 113 as monitor light.
Consequently, in the optical modulator 100 shown in FIG. 5, which is applied for example an NRZ (Non Return to Zero) modulation scheme, a voltage V1-V2 in FIG. 4 can be regarded a half-wavelength voltage Vn. Then, the control section 125 feedback controls a bias voltage V3 based on the value of the monitor light (refer to reference character B in FIG. 4) from the light reception section 121 such that a voltage V1 is applied to the electrode 111 when the optical output signal has the high level but another voltage V2 is applied to the electrode 111 when the optical output signal has the low level.
As a modulation method of an optical signal, various methods such as duo binary, DPSK (Differential Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) methods are available in addition to the NRZ method described above. However, in all methods, a photodiode having a comparatively large light reception diameter is disposed as the light reception section 121 at the substantially center of a monitor light beam so that the monitor light is received. Consequently, the light amount which can be received by the light reception section 121 is secured and the allowance of the mounting position of the photodiode with reference to the light reception amount by the photodiode is moderated.
In particular, while the output signal light is light formed by picking up only light within a reduced area at the center of the waveguide using an optical fiber not shown connected to the outgoing end face of the substrate 191, the monitor light is received over a wide area by the light reception section 121.
FIG. 20 is a view showing an example of a configuration of an optical modulator 200 as an optical device to which the method (a-ii) described above wherein the main signal is branched and monitored using a half mirror or the like arranged for the signal light after it is outputted from an optical substrate is applied as a method for monitoring output signal light.
In the optical modulator 200 shown in FIG. 20, an MZ type optical waveguide 210 from which the monitoring optical waveguide 106 shown in FIG. 5 is omitted is formed on a substrate 191. In particular, the MZ type optical waveguide 210 includes an input waveguide 101 for receiving input light, an MMI 102 connected to the input waveguide 101 for branching the input waveguide 101, two arm waveguides 103 branched at the MMI 102, another MMI 104 connected to the two arm waveguides 103 for coupling the two arm waveguides 103, and an output optical waveguide 105 connected to the MMI 104 after the two arm waveguides 103 are coupled by the MMI 104.
The optical modulator 200 further includes a half mirror 231 for branching part of light propagated along the output optical waveguide 105 and outputted from an outgoing end face 210a of the substrate 191. The optical modulator 200 further includes an electrode 111, a light reception section 121, a modulation electric signal generation section 123, a bias voltage generation section 124 and a control section 125 similar to those of the optical modulator 100 shown in FIG. 5. It is to be noted that a voltage signal generation section 122 for generating a voltage signal for the electrode 111 is formed from the modulation electric signal generation section 123 and the bias voltage generation section 124. In FIG. 20, the light reception section 121 is disposed so as to receive one of light fluxes branched by the half mirror 231, and the other one of the light fluxes branched by the half mirror 231 is coupled with an optical fiber through a lens or the like not shown.
Consequently, since, in the optical modulator 200 shown in FIG. 20, light corresponding to the output signal light is fetched as the monitor light by the light reception section 121, similarly as in the optical modulator 100 shown in FIG. 5, the control section 125 performs feedback control of the bias voltage of the bias voltage generation section 124 based on the value of the monitor light from the light reception section 121. It is to be noted that, in the optical modulator 200 shown in FIG. 20, the half mirror 231 is applied in order to fetch the monitor light by means of the light reception section 121. Therefore, in the optical modulator 200, it can be supposed to be more important than in the optical modulator 100 shown in FIG. 5 to apply a photodiode having a comparatively large light reception diameter as the light reception section 121 in order to secure the light reception amount.
It is to be noted that techniques relating to the present invention are disclosed, for example, in the following Patent Documents 1 to 3.
[Patent Document 1] Japanese Patent Laid-Open No. 2003-270468
[Patent Document 2] Japanese Patent Laid-Open No. HEI 11-52158
[Patent Document 3] Japanese Patent Laid-Open No. 2006-91785
However, while, in the operation point control of such an optical modulator 100 as described above with reference to FIG. 5 to which an optical device is applied, a in the deviation nΠ+α in phase difference between the output signal light and the monitor light preferably is substantially zero, for example, as indicated by reference character a in FIG. 6, the value α does not fully become zero. The phase deviation α is called bias shift (bias shift is hereinafter referred to sometimes simply as BS). It is to be noted that FIG. 6 is a view illustrating the bias shift a where the monitor light is fetched using the method (b-ii) described hereinabove.
While the output signal light and the monitor light are propagated along and outputted from the output optical waveguide 105 and the monitoring optical waveguide 106, respectively, phase variation arises from mixing between 0th-order mode light and first-order mode light in the process of the light propagation in the waveguides 105 and 106. The deviation in phase variation which appears with light propagated along the output optical waveguide 105 and the monitoring optical waveguide 106 makes a cause of generation of the bias shift described above.
Further, if the amount of the bias shift described above increases, then the transmission quality of light degrades. In particular, the variation of the monitor signal to be utilized as a reference in the feedback control of the bias voltage is displaced from the variation of the output signal light. Therefore, the output signal light is controlled at a bias point deviated from an optimum point of the feedback control.
Since, in the case of the NRZ modulation method described above, the bit rate of the modulation signal handled is comparatively low, and the influence of the bias shift on the transmission quality is comparatively small. However, in such modulation methods as the duo binary, DPSK, DQPSK methods and so forth developed together with increase of the bit rate of modulation data in recent years, it is supposed that, even if the amount of the bias shift is very small, the influence thereof on the transmission quality increases.
FIGS. 7(a) to 7(d) are views illustrating, as an example, phase variation of monitor light which causes appearance of the bias shift a in the optical modulator 100 described above with reference to FIG. 5.
Components of 0th-order mode light and first-order mode light are included dominantly in the monitor light outputted from the side face of the substrate 191 after reflection by the reflection groove 113 and received by the light reception section 121. If the position of the outgoing end face at which the light reflected by the reflection groove 113 is outputted from the substrate 191 is placed at an X-coordinate, then the monitor light has field intensity distributions of the 0th-order mode light and the first-order mode light indicated by reference characters A1 and A2 in FIG. 7(a), respectively. Further, while, as indicated by reference character B1 in FIG. 7(b), a bias shift component described above is not included in the 0th-order mode light, as indicated by reference character B2, a fixed phase variation amount component which does not rely upon the end face position is included in the first-order mode light.
The 0th-order mode light and the first-order mode light exhibits a light intensity which differs depending upon the outgoing position thereof as indicated by reference characters C1 and C2 in FIG. 7(c). Also the phases of the 0th-order mode light and the first-order mode light differ. The light actually outputted from the end face at the end face position X is light produced by interference of the 0th-order mode light and the first-order mode light interfere with each other and has the intensity indicated by reference character C3 in FIG. 7(c).
In particular, since the intensities and phases of the 0th-order mode light and the first-order mode light differ depending upon the end face position X, when the first-order mode light and the 0th-order mode light interfere with each other, also the interference mode differs depending upon the end face position X. Therefore, even if the first-order mode light has a fixed bias shift amount which does not rely upon the end face position X and the 0th-order mode light does not have a phase variation amount, variation of the phase variation amount, that is, a spatial distribution, appears depending upon the end face position X as seen in FIG. 7(d).
The end face position X described above with reference to FIGS. 7(a) to 7(d) can be matched with the center position of the light reception face of the photodiode which serves as the light reception section 121. In particular, even if a photodiode having a comparatively large light reception diameter is disposed, such a spatial distribution of the phase variation amount as shown in FIG. 7(d) appears depending upon the center position of the light reception face for receiving the monitor light. Therefore, the value of the phase variation amount is varied by a very small displacement of the mounting position of the photodiode, which gives rise to variation of the influence on the transmission quality.
As described above, since a photodiode having a comparatively large light reception diameter is applied as the light reception section 121, the light intensity necessary for monitoring can be obtained without performing strict alignment of the position of the light reception face of the photodiode. However, the light reception section 121 receives also light produced by interference of the 0th-order mode light, which has no bias shift component, from within light outputted from the end face with the first-order mode light which has a bias shift component. Therefore, such a bias shift as described above appears, and it is difficult to grasp the bias shift amount only from the monitor light.
Since, in the output optical waveguide 105, some waveguide length is secured normally and higher-order mode light is eliminated at the outgoing point of time, the phase variation which arises when the light propagated along the monitoring optical waveguide 106 described above is received by the light reception section 121 does not appear in the output signal light outputted from the output optical waveguide 105. Therefore, if the phase variation amount regarding the output signal light outputted from the output optical waveguide 105 is ignored, then the phase variation amount of the light propagated along the monitoring optical waveguide 106 can be considered as it is as the bias shift amount.
Such a bias shift as described above with reference to FIG. 4 arises from a factor that light produced by interference of the 0th-order mode light and the higher-order mode light is received by the photodiode in this manner. Further, also the fact that mixing occurs between the 0th-order mode light and the higher-order mode light of the propagated light depending upon the bent pattern of an optical waveguide for introducing monitoring light and shifts the phase of the 0th-order mode light itself makes a factor of appearance of a bias shift if the mixing manners of the 0th-order mode light and the higher-order mode light are different from each other.
Further, also in operation point control of such an optical modulator 200 as an optical device as described hereinabove with reference to FIG. 20, some component of light monitored by means of the light reception section 121 which receives one of light fluxes branched by the half mirror 231 may possibly have the phase deviation α which makes a bias shift in the half-wavelength voltage Vn with respect to the output light signal branched by the half mirror 231 and to be coupled to an optical fiber not shown as seen in FIG. 21 (refer to a deviation ΔV of the operation point voltage in FIG. 21).
In particular, while, in the optical modulator 200 shown in FIG. 20, light fluxes whose phase is modulated by the arm waveguides 103 are multiplexed by the MMI 104 and coupled to the output optical waveguide 105, the modulated light includes not only the 0th-order mode light to which original suitable modulation is applied but also the higher-order mode light whose phase is displaced from that of the original modulation, for example, as shown in FIG. 22.
Generally, the output optical waveguide 105 is designed such that some degree of length is secured to cut off higher-order mode light. However, under such various constraints on the design that the length of the substrate 191 is restricted for downsizing of the device and it is demanded to secure the length of the arm waveguides 103 required for reduction of the voltage, it is difficult to implement the output optical waveguide 105 which fully removes higher-order mode light. Therefore, not a little higher-order mode light remains in the output optical waveguide 105.
Where a photodiode whose light reception area is large is applied as described above as the light reception section 121 for receiving the light in which higher-order mode light remain in this manner, it receives not only the 0th-order mode light component but also the remaining higher-order mode light. Therefore, a bias shift similar to that in the case described hereinabove with reference to FIGS. 7(a) to 7(d) appears in the component monitored by the light reception section 121.
It is to be noted that all of the techniques disclosed in Patent Documents 1 to 3 suppress optical transmission of higher-order mode light but none of Patent Documents 1 to 3 discloses or suggests a configuration for suppressing the bias shift.